1. Field of the Invention
The present invention relates to a method and a device to control those functions of a gas chromatograph which depend on the advancement stage of a sample analysis, and particularly the functions which can be correlated to the presence of eluent in the separation column or in the precolumn.
2. Description of the Prior Art
As it is known, in a gas chromatographic analysis a number of the gas chromatograph functions depend on the advancement stage of the analysis itself. Examples of said functions are the on/off switching of valves in an HPLC/GC interface of the so-called "loop" type, switching which depends on the sample complete introduction; or the rise of the oven temperature after the eluent complete evaporation in analyses using a precolumn (retention gap).
Another example, the following description will refer to, is represented by the commutation of the regulating valves of an exhaust duct, or splitter, mounted on a point whatsoever of the sample elution run, between the injector and the detector, in a gas chromatograph used to analyze large volumes of samples (e.g., samples containing up to several ml of eluent). As it is known, in said analyses the splitter has the function of avoiding that the whole considerable amount of eluent (solvent, supercritical fluid or the like) flows through the column and/or the detector, discharging most of it before arriving, while only a small fraction of same is actually sent to the analysis or the detector.
Obviously, once the eluent has passed, all (or almost) the remaining part of the sample, which in the meantime has started to elute, will be sent to the detector and its components analyzed: a switching valve working on the splitter is provided for this purpose. Of course, before operating said valve, it is necessary to ensure the eluent has practically all passed through the precolumn or column, which can be done only by monitoring the solvent peak on the detector during the first part of the injection, or by performing a test injection and determining how long the solvent takes to elute completely. If the volume of the injected sample varies, said test injection must be repeated, in that the solvent elution time varies as well. Anyhow, in both cases, the time the operator requires for said controls is too long. Furthermore the time necessary for said operations is never exactly the same; if for instance it is considered the time elapsed between the sample injection and the eluent complete evaporation, it can be noticed that said time is conditioned by a lot of factors which render the moment of evaporation end not reproducible with sufficient precision. Similar problems arise in the other aforementioned examples of the gas chromatograph controllable functions. There is therefore the need to automate the control of those gas chromatographic functions which depend on the advancement stage of the analysis itself, but which cannot be precisely correlated to the time elapsed from the beginning of the analysis.